Abstract:
PLEASE NOTE: This is a preliminary release of the Asian 30 arcsecond DEM. Constructive comments from users of this data set are greatly appreciated. Please contact Kris Verdin (kverdin@edcserver1.cr.usgs.gov) or Sue Jenson (jenson@edcserver1.cr.usgs.gov) with your comments.

Data Set Assembly

A Digital Elevation Model (DEM) consists of a sampled array of elevations for ground positions that are ... normally spaced at regular intervals. The EROS Data Center's (EDC) 30 arcsecond DEM project includes generation of 30 arcsecond DEM data for the entire world. These data are being made available to the public via anonymous ftp as they become available. As major geographic regions are completed, the data, along with various derivative data sets, will be published on CD-ROM. As of December 1995, Asia, Europe, North America, and Africa are complete and available for distribution. The South American data set is under development, as are Australia, Antarctica and Indonesia.

The Asian 30 arcsecond DEM was compiled from several data sources. The primary source was a generalization of the Level 1 Digital Terrain Elevation Data. Digital Terrain Elevation Data (DTED) is a 1 degree by 1 degree dataset produced by the US Defense Mapping Agency (DMA) that contains digital data in the form of a uniform matrix of terrain elevation values for most parts of the world. It was originally designed to provide basic quantitative data for military training, planning and operating systems that require terrain elevation, slope and related information. This includes applications such as modeling the influence of terrain on radar line-of-sight, automatic height determination, terrain modeling etc.

The EROS Data Center's Digital Elevation Modeling project, that includes generation of 30 arc second data for the entire world, employed the DTED-Level 1 data in developing a 30 arc second DEM of Asia. The DTED data cover over 90% of the landmass of Asia. The elevation data for the areas of Asia which are not covered by the DTED data were developed using the 1:1,000,000 scale Digital Chart of the World (DCW) mapping.

The point and contour hypsography coverages were processed into a 30 arcsecond DEM using the ANUDEM gridding software. This software utilitized the DCW hydrography information along with the hypsograpy to generate a hydrologically realistic DEM. This technique was used to produce the 30 arcsecond DEM in all areas not covered by existing, distributable DEMs.

The data generated in this fashion were merged with the existing resampled data using various mosaicing techniques to minimize the discontinuities between the data sources.

Spatial extent of coverage

The 30 arcsecond Asian DEM complements the previously released 30 arcsecond DEM of Europe to provide 30 arcsecond cover of the entire Eurasian landmass. The largest part of the Asian DEM extends from 70E to 180E longitude and 0N to 85N latitude. A small portion covering the Chukchi Peninsula which extends across the international date line extends from 180W to 168W longitude and 56N to 72N latitude. To fill out the Eurasian continent, additional coverage extending from the Middle East to India extends from 32E to 70E longitude and 12N to 35N latitude.

Data sources and characteristics

DTED-1 has been derived from a number of cartographic and photographic sources, and by utilizing various techniques. Level 1 within DTED refers to the specific spatial resolution model used, which in this case, posts every 3 arcsecond in the original data.

DTED-1 is set to the Geographic Coordinate Reference System, and the following datum specifications:

The standard file size is a 1 degree by 1 degree geographic cell referenced by its south-west corner coordinates. Each of the elevation data records contain 1201 elevation values in meters along a single meridian, and a cell can have 201 to 1201 elevation data records depending on the latitudinal location. The spacing of elevations is according to the following:

The original 3 arcsecond DTED-1 data for Asia exists on Compact Discs (CD-ROM) that use the standard volume and file structure - ISO 9660. These data were converted to a Grid format using Arc/Info's Grid module. These full resolution 3 arcsecond grids were resampled to 30 arcsecond using Grid's "aggregate" function that generates a reduced resolution version of a grid:

The cell size of 3.000 in the original grids was aggregated using a cell factor of 10 and median as the aggregation type. The raster image contains representative elevation values for each pixel in meters above MSL.

Data Distribution

In order to facilitate distribution of the data, the Asian 30 arcsecond DEM has been divided into six pieces as follows:

Data are distributed as compressed BIL images, 16-bit straight raster image files in a latitude/longitude system. The image files were compressed using the GNU 'gzip' utility and can be identified by the .bil.gz extensions. These files must be uncompressed before use. They can be uncompressed with the gzip utilities or if you do not have access to gzip, the FTP server can uncompress the files as you retrieve them. To do this, simply leave off the ".gz" extension when retrieving the file (NOTE: This option is not available through MOSAIC). For example, to retrieve the file "asia_30_dem1.bil.gz" without compression just use "get asia_30_dem1.bil". Please note that the uncompressed files are typically many times larger than the compressed versions and therefore will take many times longer to transmit. If you would like to obtain the gzip program, it is available via anonymous FTP at the following sites:

prep.ai.mit.edu:/pub/gnu wuarchive.wustl.edu:/systems/gnu

Each image file is accompaned by five ancillary files (header file, world file, statistics file, coordinate file, and data descriptor record). The DEM Name makes up the prefix of each file. The three digit suffix varies depending on the file type. The formats of the ancillary files are described below:

Header File (.hdr)

The header file can be used as input to the ARC/INFO Image Integration application. Information contained within this file can also be used with other image display packages such as IMDISP. The following list identifies the keywords used in the header file (ARC/INFO User's Guide, Image Integration, 5-4 to 5-10):

byteorder: byte order in which image pixel values are stored M = Motorola byte order (high order byte first--Sun, HP, etc.) layout: organization of the bands in the file bil = band interleaved by line (note: single band image. This field is required for image integration in ARC/INFO. nrows: number of rows in the image ncols: number of columns in the image nbands: number of spectral bands in the image (1) nbits: number of bits per pixel (16) bandrowbytes: number of bytes per band per row (twice the number of columns) totalrowbytes: total number of bytes of data per row (twice the number of

columns) bandgapbytes the number of bytes between bands in a BSQ format image (0) nodata value used for masking purposes (will be ignored by the Image Integration application; it is not a standard ARC/INFO keyword) ulxmap: longitude of the center of the upper-left pixel ulymap latitude of the center of the upper-left pixel xdim x dimension of a pixel in geographic units (decimal degrees) ydim y dimension of a pixel in geographic units (decimal degrees)

The world file can be used for image-to-world transformation when displaying an image using the ARC/INFO Geographic Information System (GIS) Image Integration routine. The following is an example of a world file with a description of each record (ARC/INFO User's Guide, Image Integration, 2-3 to 2-5):

Example World file: Description

0.0083333333333333 x dimension of a pixel (decimal degrees) 0.00000000000000 rotation term (will always be zero) 0.00000000000000 rotation term (will always be zero) -0.0083333333333333 negative y-dimension of a pixel (in decimal degrees) 70.00416666666666 longitude of the center of the upper-left pixel 42.49583333333333 latitude of the center of the upper-left pixel

The Statistics File (.stx)

The statistics file contains the band number, the minimum pixel value (elevation expressed in meters above mean sea level), the maximum pixel value, the mean, and the standard deviation.

Example Statistics File:

1 -9999 9715 -3482.2 5841.1

The Coordinate File (.tik)

The coordinate file is named after the image with a .tik extension. The following list identifies the keywords used in the coordinate file.

ulxmap: longitude of the center of the upper-left pixel ulymap: latitude of the center of the upper-left pixel urxmap: longitude of the center of the upper-right-pixel urymap: latitude of the center of the upper-right-pixel llxmap: longitude of the center of the lower-left-pixel llymap: latitude of the center of the lower-left-pixel lrxmap: longitude of the center of the lower-right-pixel lrymap: latitude of the center of the lower-right-pixel xdim: x dimension of a pixel in geographic units (decimal degrees) ydim: y dimension of a pixel in geographic units (decimal degrees)

This file has been included to allow the image to be read directly by EDC's LAS software. To take advantage of this, the image file extension will have to be renamed from bil to img. This file contains the information contained in the other ancillary data files.

Along with access via ftp transfer, customers can receive these data sets through a tape order. If the data were obtained through a tape order, this README.txt file will be the first record on the tape, followed by the five ancillary files and corresponding images. Each ancillary file contains one record, while for the image, each row of data is one record long.

For the areas of the Asian landmass lacking DTED data, the ANUDEM software was used to grid a DEM from the 1:1,000,000 scale DCW data set. The map source for the DCW database is the U.S. Defense Mapping Agency (DMA)'s Operational Navigation Chart (ONC) series. This is the largest scale map series that provides consistent, continuous global coverage of essential basemap features.

A brief description of the techniques used to produce a DEM from the DCW data set is included here.

Drainage Orientation:

Initially, DCW files are stored as vector contours (hypsography) and hydrography. Using automated stream orientation procedures, contour line and point data are compared to the hydrographic network to determine drainage direction. Drainage lines are oriented in the direction of flow, so that they are ordered from the highest point to the lowest. After the stream orientation process is complete, the resulting output is verified and edited to ensure that all streams are represented as flowing downhill.

DEM Generation:

Once data are pre-processed, they are used as input into the Australian National University Digital Elevation Model (ANUDEM) generation program developed by Michael Hutchinson, Centre for Resources and Environmental Studies, Australian National University. This program first reads input elevations, windows the data to the specified map limits, then generates a grid at 30 arcsecond intervals. Elevation data are generalized by accepting a maximum of 4 data points per grid cell and discarding any remaining points. Contour and hydrography line data are generalized by accepting a maximum of one line per grid cell. The program then employs a multi-grid method that calculates grids at successively finer resolutions until the specified grid resolution is achieved. During this process, drainage conditions are imposed to remove sinks where possible. Values at grid points not occupied by data points are calculated by Gauss-Seidel iteration with over relaxation (SOR method) subject to an appropriate roughness penalty and ordered chain constraints. The ordered chain constraints are obtained from the stream line, sink point, and contour line data and through automatic drainage enforcement as calculated by the program. Starting values for the first coarse grid resolution are calculated from a least squares plane fit to the data points. Values for each succeeding grid are linearly interpolated from the preceding grid (M.F. Hutchinson, unpub. data, 1991).

Data Characteristics

Spatial Resolution

Spacing of the elevations along and between each sample is 30 arcseconds (approximately 1 km). The horizontal datum is WGS84. Elevation values are expressed in meters above mean sea level.

Accuracy

The absolute accuracy of the DCW vector information is 2000 meters circular error (horizontal) and + or - 650 meters linear error (vertical) at 90-percent confidence as defined by the Defense Mapping Agency (DMA). The grid generated from these data will be no more accurate than this source. The accuracy for the grid has not been measured or calculated.

Data Availability Procedures for Obtaining Data

Data sets for Asia, Europe, North America, and Africa are available through an Internet anonymous File Transfer Protocol (FTP) account at the EDC. These 30 arcsecond DEMs are available at no charge.

To access this account: 1. FTP to 152.61.128.6 (edcftp.cr.usgs.gov) 2. Enter "anonymous" at the Name prompt. 3. Enter your email address at the Password prompt. 4. Change (cd) to the "pub/data/30ASDCWDEM" subdirectory. 5. The files are located under the subdirectories: EUROPE NORTH_AMERICA AFRICA ASIA 6. Files are named after the image with the following extensions: *.bil.gz = compressed image file *.blw = world file *.hdr = header file *.stx = statistics file *.tik = coordinate file *.ddr = data descriptor record 7. Enter "binary" to set the transfer type 8. Use get or mget to retrieve the desired files.

Orders for tape copies can be placed through the GLIS Inventory SUMMARY RESULTS screen or by contacting the EROS Data Center, Customer Services section, at the following address:

Customer Services can also be contacted for information about DCW DEM data that are in progress. As additional geographic areas become available they will be announced in the GLIS News.

Applications and Related Data Sets

Moderate resolution (100 meter to 1 kilometer) topographic data have applications in many diverse land science fields such as geology and geophysics, ecology, soil science, botany, and glaciology. Topographic data are also critical to procedures used for correcting and/or presenting remotely sensed satellite and other global data.

The 30 arc-second DEM data, produced for use in conducting large-area studies, have been generated at a resolution which is compatible with the Advanced Very High Resolution Radiometer (AVHRR) sensor.

Attached Raster(s): Member_ID: 2 Raster Name: COVERS AN AREA OF LATITUDE 42.5N-85N AND LONGITUDE 70E-125E Raster Projection: GEOGRAPHIC Raster Resolution: 92 Number of Rows: 5100 Number of Columns: 6600 Number of Bits: 16 Raster 30 ARC-SECOND DEM DATA ARE ALSO AVAILABLE FROM THE USGS/EROS DATA

Attached Raster(s): Member_ID: 3 Raster Name: COVERS AN AREA OF LATITUDE 0-42.5N AND LONGITUDE 125E-180E Raster Projection: GEOGRAPHIC Raster Resolution: 92 Number of Rows: 5100 Number of Columns: 6600 Number of Bits: 16 Raster 30 ARC-SECOND DEM DATA ARE ALSO AVAILABLE FROM THE USGS/EROS DATA

Attached Raster(s): Member_ID: 4 Raster Name: COVERS AN AREA OF LATITUDE 42.5N-85N AND LONGITUDE 125E-180E Raster Projection: GEOGRAPHIC Raster Resolution: 92 Number of Rows: 5100 Number of Columns: 6600 Number of Bits: 16 Raster 30 ARC-SECOND DEM DATA ARE ALSO AVAILABLE FROM THE USGS/EROS DATA

Attached Raster(s): Member_ID: 5 Raster Name: COVERS AN AREA OF LATITUDE 56N-72N AND LONGITUDE 180W-168W Raster Projection: GEOGRAPHIC Raster Resolution: 92 Number of Rows: 1920 Number of Columns: 1440 Number of Bits: 16 Raster 30 ARC-SECOND DEM DATA ARE ALSO AVAILABLE FROM THE USGS/EROS DATA

Attached Raster(s): Member_ID: 6 Raster Name: COVERS AN AREA OF LATITUDE 12N-35N AND LONGITUDE 32E-70E Raster Projection: GEOGRAPHIC Raster Resolution: 92 Number of Rows: 2760 Number of Columns: 4560 Number of Bits: 16 Raster 30 ARC-SECONDDEM DATA ARE ALSO AVAILABLE FROM THE USGS/EROS DATA

Attached Raster(s): Member_ID: 1 Raster Name: COVERS AN AREA OF LATITUDE 0-42.5N AND LONGITUDE 70E-125E Raster Projection: GEOGRAPHIC Raster Resolution: 92 Number of Rows: 5100 Number of Columns: 6600 Number of Bits: 16 Raster 30 ARC-SECOND DEM DATA ARE ALSO AVAILABLE FROM THE USGS/EROS DATA